In U.S. Pat. No. 5,448,582, issued Sep. 5, 1995, entitled "Optical Sources Having a Strongly Scattering Gain Medium Providing Laser-Like Action", the inventor disclosed a multi-phase gain medium including an emission phase (such as dye molecules) and a scattering phase (such as TiO.sub.2). A third, matrix phase may also be provided in some embodiments. Suitable materials for the matrix phase include solvents, glasses and polymers. The gain medium is shown to provide a laser-like spectral linewidth collapse above a certain pump pulse energy. The gain medium is disclosed to be suitable for encoding objects with multiple-wavelength codes, and to be suitable for use with a number of substrate materials, including polymers and textiles.
It is well known in the art to use security threads in paper to hinder a non-authorized production of the paper or to authenticate already manufactured paper and/or a document or currency printed on the paper. Reference in this regard can be had to the following U.S. Pat. Nos.: 5,486,022, "Security Threads Having At Least Two Security Detection Features and Security Papers Employing Same", by T. T. Crane; 4,534,398, "Security Paper", by T. T. Crane; and 4,437,935, "Method and Apparatus for Providing Security Features in Paper", by F. G. Crane, Jr.
A problem currently exists in accurately authenticating certain documents, such as currency, bank drafts, stock certificates, bonds, checks, and negotiable instruments in general. It is widely known that modern counterfeiters have access to sophisticated technology, and can reproduce nearly indistinguishable copies of currency and other documents. As a result, it has become very difficult to unambiguously authenticate a given document.